The present invention relates to an ammonia generating method according to the contact hydrolysis of urea, and a combusted exhaust gas processing method using the same.
Nitrogen oxide in the exhaust smoke discharged from the thermal electric power plants, the automobiles, the various factories, or the like is one of the main causes of the photochemical smog. As an effective eliminating method therefor, the catalytic reduction method using ammonia (NH3) as the reducing agent, has been used widely, mainly in the thermal electric power plants. Although liquid NH3 is used in the thermal electric power plants for denitration, it is difficult to adopt liquid NH3 in the denitration devices for public use, to be installed in a densely populated area. Therefore, a method of using a nitrogen compound such as urea as a safe and easily handled substitute reducing agent for generating NH3 by the decomposition can be presented. A method of executing denitration by adding a substitute reducing agent such as urea directly to the exhaust gas flue can hardly be adopted to, in particular, the denitration at a low temperature of 400xc2x0 C. or less due to problems such as the denitration reaction efficiency decline due to the poor decomposition efficiency to NH3, and generation of scales derived from the substituent reducing agent inside the flue. Accordingly, a method of using a reducing agent decomposing device filled with the reducing agent decomposing catalyst for decomposing the reducing agent so as to generate NH3, and using NH3 for the denitration has been proposed (the official gazette of Japanese Patent Application Laid Open (JP-A) No. 5-15739).
According to the method, by contacting a reducing agent comprising a solid nitrogen-containing compound such as urea, cyanuric acid, melamine, and biuret, and a heated gas (such as air) containing water vapor with a metal oxide such as alumina, silica, silica-alumina, calcia, magnesia, and titania or a reducing agent decomposing catalyst such as zeolite, the solid reducing agent is thermally decomposed and/or hydrolyzed so as to generate ammonia. In order to promote the decomposition of the reducing agent, a catalyst with a strong solid basicity is effective. Therefor, two or more metal oxides can be used in a combination, or a component such as tungsten, vanadium, iron, molybdenum, copper, cobalt, tin, nickel, chromium, sulfur, magnesium, boron, barium, lanthanum, or the like can be added to the metal oxide. The reducing catalyst is used as a pellet shaped in a spherical shape, a hollow columnar shape, and a columnar shape. The temperature necessary for the decomposition of the reducing agent differs depending on the kind thereof and the kind of the decomposing catalyst, but in order to obtain the sufficient decomposition efficiency, in the case of the above-mentioned method, heat of 350 to 500xc2x0 C. is necessary. As an example, in the case the reducing agent is urea, Japanese Patent Application Laid Open (JP-A) No. 5-15739 mentions that it is difficult to obtain NH3 with a high efficiency without having the heat of the reducing agent decomposing catalyst of at least 350xc2x0 C.
As mentioned above, in the case of using a metal oxide, or the like as the decomposition catalyst so as to decompose a solid reducing agent such as urea for generating NH3, in order to convert the reducing agent to NH3 highly efficiently, a high temperature of 350xc2x0 C. or more is needed. Since the exhaust gas temperature from the outlet of an ordinary combusting device (boiler, diesel engine, or the like) to a nitration device is 200 to 450xc2x0 C., it is in many cases difficult to maintain the temperature of the decomposing catalyst at 350xc2x0 C. or more only by the exhaust gas temperature. In particular, at the time of starting, stopping, or a low load, the exhaust gas temperature is low, and thus it is difficult to maintain the decomposing catalyst temperature at 350xc2x0 C. or more. Therefore, a reducing agent decomposing method capable of generating NH3 at a lower temperature is desired.
The present invention is to solve the problems in the conventional technique and to provide an NH3 generating method capable of executing the urea hydrolysis at a lower temperature efficiently and generating NH3 at the flue exhaust gas temperature, and a combusted exhaust gas processing method using the same.
In order to solve the above-mentioned problems, the present invention adopts the following:
(1) An ammonia generating method which comprises generating ammonia by the contact hydrolysis of urea by contacting at 200xc2x0 C. or more an aqueous solution of urea with a contact hydrolysis catalyst containing at least one selected from the group consisting of hydroxides, carbonates, and silicates of alkaline metals as the main component.
(2) The ammonia generating method according to the above-mentioned first aspect, wherein the contact hydrolysis catalyst is a contact hydrolysis catalyst containing at least one selected from the group consisting of solids of NaOH, KOH, Na2CO3, K2CO3, Na2SiO3 and K2SiO3 as the main component.
(3) The ammonia generating method according to the above-mentioned first or second aspect, wherein the contact temperature of the aqueous solution of the urea and the contact hydrolysis catalyst is 200 to 450xc2x0 C.
(4) A combusted exhaust gas processing method for denitration by passage of a combusted exhaust gas in a denitration device using ammonia, which comprises installing a reducing agent decomposing reactor provided with a contact hydrolysis catalyst layer containing at least one selected from the group consisting of hydroxides, carbonates, and silicates of alkaline metals as the main component in the front flow of the denitration device disposed at a part with a 200 to 450xc2x0 C. exhaust gas temperature in the flue, supplying an aqueous solution of urea to the reducing agent decomposing reactor for the contact hydrolysis, and adding the generated ammonia in the exhaust gas by the front flow of the denitration device for the denitration.
(5) A combusted exhaust gas processing method for denitration by passage of a combusted exhaust gas in a denitration device using ammonia, and adding ammonia for neutralizing sulfuric acid and sulfuric anhydride, which comprises installing a reducing agent decomposing reactor provided with a contact hydrolysis catalyst layer containing at least one selected from the group consisting of hydroxides, carbonates, and silicates of alkaline metals as the main component in the front flow of the denitration device disposed at a part with a 200 to 450xc2x0 C. exhaust gas temperature in the flue, supplying an aqueous solution of urea to the reducing agent decomposing reactor for the contact hydrolysis, adding the generated ammonia in the exhaust gas by the front flow of the denitration device for the denitration, and introducing a part of the ammonia to a part with a 100 to 200xc2x0 C. exhaust gas temperature in a back flow of the denitration device so as to be sprinkled in the exhaust gas for neutralizing the sulfuric acid and the sulfuric anhydride.
According to an ammonia generating method of the present invention, NH3 can be generated from urea as an easily handled solid reducing agent further easily by a reaction at a relatively low temperature. The method is preferable as an NH3 generating method in the exhaust gas process such as the exhaust smoke denitration or the neutralization of sulfuric acid or sulfuric anhydride in the exhaust smoke in a selective catalyst reduction with NH3 as the reducing agent, and other processes requiring NH3. According to the method, since NH3 can be utilized without using hardly handled liquid ammonia, the risk of the environmental pollution can be avoided in the area close to the urban district. Moreover, according to an exhaust gas processing method of the present invention, the exhaust smoke denitration or the neutralization process of sulfuric acid and sulfuric anhydride in the exhaust smoke can be executed by the selective catalytic reduction without using liquid ammonia. Moreover, since the decomposing reaction of the urea can be executed at a low temperature of about 200xc2x0 C., the exhaust gas heat can be utilized as the heat source for the reaction so that a heater needs to be used only when the exhaust gas has a low temperature at the time of starting or stopping the device or a low load drive, and thus it is an extremely efficient process.